[BUG][KOTLIN] No JsonAdapter for LocalDateTime in Moshi serializer

Created by: toXel

Description

date-time fields can't be parsed from a generated Kotlin client with default configuration. It fails with this exception: java.lang.IllegalArgumentException: Platform class java.time.LocalDateTime annotated [] requires explicit JsonAdapter to be registered

The reason for it is that in Serializer.kt.mustache Moshi only gets a JsonAdapter for the java.util.Date type but not for java.time.LocalDateTime (or org.threeten.bp.LocalDateTime when ThreeTen is used).

openapi-generator version

v3.3.1

Command line used for generation

generate -g kotlin -i https://petstore.swagger.io/v2/swagger.yaml -o swaggerClientTest

Steps to reproduce

It can be reproduced with the petstore swagger spec.

1. Generate Kotlin client with the command above.
2. Write a simple test like this:

@Test
fun getOrderById() {
    val order = StoreApi().getOrderById(1)
    assertNotNull(order)
}

3. It will fail with the respective exception.

Suggest a fix/enhancement

A workaround is to implement a JsonAdapter for the respective type and add it to Moshi in the generated Serializer class. Maybe this can be integrated in the generator.